US 2009/0002054 corresponding to JP-A-2009-11049 discloses a gate driver circuit for driving a gate of a switching device (as a load) with a constant current. In the gate driver circuit, a series circuit of a first resistor and a second resistor is connected to a power source, and a base of a PNP transistor is connected to a node between the first resistor and the second resistor. Further, a MOS transistor is connected to the second resistor. Further, a collector of the PNP transistor is connected through a third resistor to the power source, and an emitter of the PNP transistor is connected to the gate of the switching device.
In the gate driver circuit, when the MOS transistor is turned ON, the PNP transistor is turned ON so that the constant current can flow from the power source to the gate of the switching device through the third resistor and the PNP transistor. Thus, the switching device is turned ON.
However, in the gate driver circuit, the constant current flowing to the switching device can vary due to temperature dependences of a gain (i.e., amplification factor) and a forward voltage Vf of the PNP transistor. Therefore, it is difficult to ensure the accuracy of the constant current.
Further, since the magnitude of the constant current supplied to the load varies depending on types of loads, the gate driver circuit is designed for the load that needs the maximum constant current. Therefore, the cost of the gate driver circuit is increased.
US 2009/0002054 further discloses that two NPN transistors are connected in a Darlington configuration to increase driving speed. However, this configuration consumes a large amount of current, because the collector of one NPN transistor is connected to the power source.
JP-3680722 discloses another constant-current gate driver circuit for turning ON and OFF an insulated gate bipolar transistor (IGBT). In the gate drive circuit, a constant current circuit constructed with a MOSFET and a resistor is connected to each of the high side and low side of the gate of the IGBT, and a gate voltage of the MOSFET is controlled by an operational amplifier to control a current flowing to the gate of the IGBT. Then, a current flowing between a collector and an emitter of the IGBT is detected by an overcurrent detection circuit. The overcurrent detection circuit outputs a detection signal upon detection of overcurrent. The detection signal is fed back to adjust an output of the operational amplifier. Thus, the current flowing to the gate of the IGBT is controlled so that the IGBT can be protected from overcurrent.
In the gate driver circuit disclosed in JP-3680722, the constant current circuit performs a feedback control to improve accuracy of the constant current. Further, the constant current circuit is constructed with a MOSFET, a driving speed may be improved. However, JP-3680722 fails to disclose a specific configuration to improve the driving speed.